Abstract:

A humidity control method and apparatus that can be utilized to provide
humidity control within an enclosed assembly such as a disc drive. The
apparatus includes a container that is at least partially formed of a
material through which water vapor can freely move. The apparatus also
includes a humidity-controlling mixture that comprises at least one salt
and a superabsorbent polymer. The humidity-controlling mixture is
enclosed within the container.

Claims:

1. A method for maintaining a relatively constant humidity in an
enclosure, the method comprising:dissolving a free solute of a
multi-phase salt mixture in response to an increase in water vapor
pressure;precipitating dissolved solute out of solution in response to a
decrease in vapor pressure; andutilizing a superabsorbent polymer to
adsorb excess solvent in the multi-phase salt mixture.

2. The method of claim 1 wherein the multi-phase salt mixture comprises a
plurality of salts.

4. A humidity control device comprising:a container that is at least
partially formed of a material through which water vapor can freely move;
anda humidity-controlling mixture within the container, the
humidity-controlling mixture comprising:a saturated aqueous solution of a
plurality of salts in intimate contact with excess solid of the same
plurality of salts; anda super-absorbent polymer.

Description:

CROSS-REFERENCE TO RELATED APPLICATION

[0001]The present application is a Continuation of and claims priority of
U.S. patent application Ser. No. 10/970,960, filed Oct. 22, 2004 which is
based on and claims the benefit of U.S. Provisional Application
60/548,028 filed on Feb. 26, 2004 for inventors Paul A. Beatty, Robert A.
Bruce, Michael D. Ries and James H. Smith and entitled "DEVICE FOR HARD
DISK DRIVE INTERNAL HUMIDITY CONTROL."

FIELD OF THE INVENTION

[0002]The present invention relates generally to humidity control in
enclosed assemblies, and more particularly but not by limitation to
humidity control in data storage devices such as disc drives.

BACKGROUND OF THE INVENTION

[0003]Disc drives are used for data storage in modern electronic products
ranging from digital cameras to computer systems and networks. Typically,
a disc drive includes a mechanical portion, or head disc assembly (HDA),
and electronics in the form of a printed circuit board assembly (PCBA),
mounted to an outer surface of the HDA. The PCBA controls HDA functions
and provides an interface between the disc drive and its host.

[0004]Generally, a HDA comprises moving parts such as one or more magnetic
discs affixed to a spindle motor assembly for rotation at a constant
speed, an actuator assembly supporting an array of read/write heads that
traverse generally concentric data tracks radially spaced across the disc
surfaces and a voice coil motor (VCM) providing rotational motion to the
actuator assembly.

[0005]A disc drive unit typically includes a housing that protects the
workings of the drive from particulates and other contamination. Control
of the internal relative humidity (RH) within the housing of the disc
drive is desirable since the moisture content of the surrounding air
affects the performance and reliability of the head/disc interface (HDI),
a crucial part of the disc drive recording system. For example, it is
known that head-to-disc stiction and media corrosion are aggravated by
high relative humidity levels. As a further example, excessive disc wear
has been observed under conditions of very low relative humidity.

[0006]A majority of commercial hard disc drive products incorporate
elements that limit the rate of moisture exchange between drive interior
and the environment. Such elements include seals, diffusion tubes, carbon
absorbers, and desiccants. The effect of these elements is simply to
delay changes in relative humidity. They do not control relative humidity
levels to a given level, nor do they prevent eventual equilibration over
the full range of external (ambient) relative humidity (from 0 to 100%).
Commonly used methods to control RH in general commercial applications
require costly elements and/or systems (such as sensors, control
electronics and software, evaporators, condensers, etc.) that preclude
their use in hard disc drives and other such assemblies that include
moving parts.

[0007]Embodiments of the present invention provide solutions to these and
other problems, and offer other advantages over the prior art.

SUMMARY OF THE INVENTION

[0008]Disclosed is a humidity control method and apparatus that can be
utilized to provide humidity control within an enclosed assembly such as
a disc drive. The apparatus includes a container that is at least
partially formed of a material through which water vapor can freely move.
The apparatus also includes a humidity-controlling mixture that comprises
at least one salt and a superabsorbent polymer. The humidity-controlling
mixture is enclosed within the container.

[0009]Other features and benefits that characterize embodiments of the
present invention will be apparent upon reading the following detailed
description and review of the associated drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is an oblique view of a disc drive that includes a humidity
control device of the present invention.

[0011]FIG. 2 illustrates plots of test results for humidity-controlling
materials of the present invention and the prior art.

[0012]FIG. 3 is a partial cross-sectional view of an embodiment of a
humidity control device of the present invention.

[0013]FIG. 4 illustrates adsorption isotherm plots for different mixtures
of a salt and a superabsorbent polymer by weight.

[0014]FIG. 5A is a diagrammatic view of another embodiment of a humidity
control device of the present invention.

[0015]FIG. 5B is a cross-sectional view of the humidity control device of
FIG. 3-1.

[0016]FIG. 6 is a diagrammatic view of another embodiment of a humidity
control device of the present invention.

[0017]FIGS. 7 and 8 is a graphical represent of test results obtained for
a disc drive with and without a humidity control device of the present
invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0018]Disclosed is a humidity control method and apparatus for use in an
enclosed assembly. The humidity control system of the present invention
is capable of countering changes in relative humidity (RH) due to
transport of water vapor into and out of the enclosed assembly (such as a
disc drive) in order to maintain relatively constant humidity conditions
inside the drive within tight limits over the entire operating
temperature range of the drive. It should be noted that the humidity
control system of the present invention is capable of maintaining
relatively constant humidity conditions within the enclosed assembly for
a significant time period (not just for a day or two).

[0019]Although an example embodiment of the humidity control device of the
present invention is shown as being employed in a disc drive in FIG. 1,
the humidity control device of the present invention can be employed in
any enclosed system in which humidity control is desired. FIG. 1
illustrates an oblique view of a disc drive 100 in which embodiments of
the present invention are useful. Disc drive 100 includes a housing with
a base 102 and a top cover (not shown) that closes the housing to form an
enclosed assembly. The housing 102 may include a breathing hole (such as
104) that is sealed with a porous filter that allows air and humidity to
move in and out of the disc drive 100 as temperature or atmospheric
pressure changes. It should be noted that some embodiments of disc
drives, with which embodiments of the present invention are also useful,
are hermetically sealed and therefore do not include a breathing hole.
Disc drive 100 further includes a disc pack 106, which is mounted on a
spindle motor (not shown) by a disc clamp 108. Disc pack 106 includes at
least one disc, which is mounted for co-rotation in a direction indicated
by arrow 107 about central axis 109. Each disc surface has an associated
disc read/write head slider 110 which is mounted to disc drive 100 for
communication with the disc surface. In the example shown in FIG. 1,
sliders 110 are supported by suspensions 112 which are in turn attached
to track accessing arms 114 of an actuator 116. The actuator shown in
FIG. 1 is of the type known as a rotary moving coil actuator and includes
a voice coil motor (VCM), shown generally at 118. Voice coil motor 118
rotates actuator 116 with its attached read/write heads 110 about a pivot
shaft 120 to position read/write heads 110 over a desired data track
along an arcuate path 122 between a disc inner diameter 124 and a disc
outer diameter 126. Voice coil motor 118 is driven by electronics 130
based on signals generated by read/write heads 110 and a host computer
(not shown). Disc drive 100 also includes a humidity control device 105
of the present invention, which maintains relatively constant humidity
conditions inside drive 100.

[0020]As mentioned above, prior art humidity control devices, utilized in
enclosed assemblies such as disc drives, include seals, diffusion tubes,
carbon absorbers, and desiccants. The effect of these elements is simply
to delay changes in RH and not to control RH within a disc drive at a
relatively constant level.

[0021]Under the present invention, a humidity control device (such as 105)
is provided, which includes a container that is at least partially formed
of a material through which water vapor can freely move. The humidity
control device also includes a humidity-controlling mixture that
comprises at least one salt and a superabsorbent polymer. The
humidity-controlling mixture is enclosed within the container. The
humidity control device can be easily installed in an enclosed assembly
such as disc drive 100. Underlying principles upon which the design of
the humidity control device of the present invention is based are
provided below. Different embodiments of the humidity control device of
the present invention are also described further below.

[0022]Consider a closed system comprised of an aqueous solution of a
relatively non-volatile chemical species (or multiple species) and humid
air. Over time, this system will come to equilibrium in which the partial
pressure of water vapor in the air is completely determined by the
temperature, total pressure, and the solute concentration. In the
dilute-solution limit, the partial pressure of water vapor above the
solution decreases linearly with increasing solute concentration (the
mole fraction of solute), under constant temperature and pressure
conditions. This phenomenon is known as Raoult's law. In general, there
will be a departure from Raoult's law as the concentration of solute
increases from the dilution limit to moderate or high levels; however,
there is still a definite monotonic relationship between the solute
concentration in the solution and the partial pressure of water vapor in
the air at all solute concentration levels.

[0023]The amount of a non-volatile component (solute) that will dissolve
in water is often limited. In such a case, as more and more solute is
added to a given volume of water, a point will be reached when further
solute will not dissolve and some pure solute will be present as a
distinct phase. This condition is known as saturation. The amount of
solute that can be dissolved in water (the solubility limit) depends on
the temperature and on the chemical composition of the solute.

[0024]If, in the closed system, the solution is a saturated solution,
there will be a three-way equilibrium among the partial pressure of water
vapor in the air, the saturated solution of the solute dissolved in the
water, and the pure solute present as a distinct, pure phase. In this
case, the concentration of dissolved solute and the partial pressure of
water vapor are not arbitrary but locked to specific values. This
equilibrium state is stable, that is, the system will respond to
perturbations by compensating changes in the opposing direction.
Specifically, if the water vapor partial pressure in the closed system
were increased by some artificial means, the solution would capture some
water vapor from the air and dissolve more of the free solute. In this
way, the partial pressure of water in the air and the concentration of
solute would be driven back towards their original levels. An artificial
decrease in the water vapor partial pressure would bring about the
reverse process with some solute precipitating out of the solution and
some liquid water evaporating to increase the water vapor partial
pressure. In such a closed equilibrium system, the partial pressure of
water vapor in the air is held to a specific value with little variation
at substantially constant temperature.

[0025]Relative humidity (RH) is a direct function of the partial pressure
of water vapor in the air. Therefore, the RH level of a closed
equilibrium system comprised of humid air, aqueous solution, and free
solute is fixed at a specific value. This RH value depends only on
temperature and the solute used. (The dependence of equilibrium RH on
total pressure is negligible). Below is a table (Table 1) for equilibrium
humidity levels for saturated aqueous solutions of various salts at 25
degrees Celsius (° C.).

[0026]These equilibrium RH levels are relatively insensitive to
temperature. To illustrate this point, a saturated solution of MgCl2
in water will control the relative humidity to 30.5% at 0° C. and
to 33.7% at 50° C. This represents a very modest RH swing over a
broad temperature range. This is an advantage since, in a practical
device, maintenance of a constant level of relative humidity over the
entire operating temperature range is desired. The average operating
temperature range of a disc drive is between about -5° C. and
55° C. and therefore RH levels within this temperature range can
be controlled effectively using the present invention. It should be noted
that suitable humidity controlling mixtures can also be prepared, using
the above principles, to provide humidity control for larger operating
temperature ranges (-40° C. to 80° C., for example).

[0027]As mentioned above, in addition to at least one salt, the
humidity-controlling mixture of the present invention also includes a
superabsorbent polymer (such as polyacrylic acid (PAA) or polyacrylamide
(PAM)). Advantages of using a mixture of a salt and a superabsorbent
polymer include: 1) the equilibrium RH above the saturated solution
remains constant as long as the solution remains saturated, 2) the
adsorption of water over all RH ranges above the equilibrium RH of the
unsaturated salt is very high, 3) as the humidity approaches 100%, the
capacity of the superabsorbent polymer to adsorb water vapor drastically
increases (goes above about 95%) and 4) the superabsorbent polymer
eliminates the puddle formed when the salt adsorbs water.

[0028]FIG. 2 illustrates plots of test results contrasting the
humidity-controlling materials of the present invention and the prior
art. As can be seen in FIG. 2, plots 202 and 204 represent adsorption
(ads) and desorption (desorp) isotherms, respectively, for a PAA and
K2CO3 mixture as embodied in the present invention. Plots 206
and 208 represent adsorption and desorption isotherms, respectively, for
silica gel (prior art). From FIG. 2, it is clear that the
humidity-absorption capability of the mixture of the present invention is
substantially greater than that of the prior art silica gel.

[0029]Different embodiments of humidity control devices, designed based
upon the above principles, are described below in connection with FIGS.
3, 5 and 6.

[0030]FIG. 3 is a partial cross-sectional view of an embodiment of a
humidity control device 300 of the present invention. The same reference
numerals are used to represent the same or similar elements in the
different embodiments of the humidity control device. In FIG. 3, the
container of humidity control device 300 is a pouch 302 formed
substantially completely of a vapor-permeable fabric 306 such as
polytetrafluoroethylene (PTFE). Humidity-controlling mixture 304 is
included within pouch 302. In embodiments of the present invention,
humidity-controlling material 304 is a mixture of at least one salt and a
superabsorbent polymer. The at least one salt may be selected from salts
such as magnesium chloride, potassium carbonate, sodium chloride and
potassium sulfate (Table 1 above). As mentioned above, examples of
superabsorbent polymers include PAA and PAM. As can be seen in FIG. 3,
humidity control device 300 also includes a mounting element 308 for
mounting device 300 within an enclosed assembly such as disc drive 100.
In some embodiments, mounting element 308 is an adhesive layer. Adhesive
layer 308 may be a pressure sensitive adhesive or VELCRO® mounting
or, in general, any type of hook and loop mounting mechanism may be
utilized. In other embodiments, mechanical means for attaching the
container (screws, clamps, clips, interference fits, wedges, etc.) may be
employed as element 308.

[0031]In a disc drive application, the saturated solution in the humidity
control device would counter changes in RH due to transport of water
vapor into or out of the disc drive housing in order to maintain constant
humidity conditions inside within tight limits over the entire operating
temperature range. It should be noted that both intentional and
unintentional paths for ongoing ingress or egress of moisture are usually
present in a disc drive. Diffusion through a port in the disc drive and
permeation through seals, gaskets, etc., are examples of how moisture can
reach the drive interior. Given a particular head/disc interface (HDI)
design, an appropriate solute species that gives the desired RH level for
that design is selected.

[0032]It should be noted that a special saturated solution need not be
prepared for application in a disc drive; within a broad range of
conditions, the system will equilibrate to an appropriate RH level
regardless of the initial condition of the material. For example, a mass
of dry salt would absorb water from the air and form a puddle of
saturated solution that would control the disc drive internal RH to the
desired level. Moreover, some anhydrous salts, such as CaCl2 may
also undergo an irreversible hydration process (at a standard operating
temperature of the disc drive) that would absorb yet more water from the
air. This latter process may aid in cases where hard disc drives are
stored for long periods in hot, wet conditions where extreme protection
from moisture is desired.

[0033]As mentioned above, humidity-controlling mixture 204 includes a salt
and a superabsorbent polymer. The salt and polymer are typically
intimately mixed. One technique for intimately mixing the salt and the
polymer includes dissolving the salt in water, adding the polymer, which
rapidly adsorbs the salt solution, and then drying the mixture. In some
embodiments, the raw dry salt and polymer powders are mixed. Depending
upon the type of application, a 30/70, 40/60, 50/50, 60/40 or 70/30
mixture of the polymer and the salt by weight may be utilized. Although
the above mixtures of the polymer and the salt by weight have been found
to be suitable as a result of tests that were carried out, it should be
noted that any suitable mixture of the polymer and the salt may be
utilized. Adsorption isotherms of different mixtures of PAA and K2CO3 by
weight are shown in FIG. 4.

[0034]In some embodiments of the present invention, to improve
performance, a mixture of multiple salts (potassium carbonate and
potassium bicarbonate, for example) and the superabsorbent polymer may be
utilized. It should be noted that utilizing potassium carbonate, or any
other carbonate, also provides a very high capacity for absorbing acid
gases, such as HCl, SO2, NOx, etc. Therefore, such mixtures help
absorb contaminants (such as H2S) that may be outgassed due to
interaction between components within the disc drive. They also help
absorb similar contaminants that may enter from outside the drive.

[0035]In the embodiment of the humidity control device shown in FIG. 3, as
mentioned above, the container of device 300 is formed substantially
completely of vapor-permeable membrane 306. However, in some embodiments,
the container may be formed of an impermeable material with a window (or
small portion) formed of a vapor-permeable membrane. FIGS. 5A and 5B are
diagrammatic and cross-sectional views, respectively, of one such
embodiment. Here, humidity control device 500 is a box including a
machined or molded portion 501 that is sealed with a permeable membrane
or fabric 502, such as PTFE. Membrane 502 forms the top of the box and
portion 501 forms side walls 504, and bottom 506, of the box. Portion 501
may be formed of plastic, for example. As can be seen in FIG. 5B, the
interior of humidity control device 500 includes humidity-controlling
mixture 505, which comprises a salt and a superabsorbent polymer. A
mounting element (not shown in FIGS. 5A and 5B) may be attached to bottom
506 of humidity control device 500 for mounting in an enclosed assembly
such as disc drive 100.

[0036]Additional features might be incorporated into the above-described
embodiments to enhance the overall functionality of the humidity control
device. In some embodiments, side walls 504 may be formed of an elastic
material to accommodate changes in volume within humidity control device
500 due to condensation of water vapor into and/or evaporation of water
out of humidity-controlling mixture 505 within humidity control device
500. Humidity control device 500, described above and shown in FIGS. 5A
and 5B, includes a single vapor-permeable membrane or patch 502 that
forms the top of the container or box. Here, there may be the potential
for stratification of the solution by gravity. For some container
orientations, condensation of water vapor may only occur near the liquid
free surface, giving a reduced concentration of solute, and hence reduced
capacity for RH control, there. This problem may be avoided by having
vapor-permeable membrane patches located on various faces of the
container that would allow water to be absorbed into the solution away
from the free surface. FIG. 6 shows a diagrammatic view of a humidity
control device 600 that includes vapor-permeable membrane 502 and an
additional vapor-permeable patch 602 on a side wall (such as 504 (FIG.
5)). Convection currents driven by solution density gradients in the
system of FIG. 6 would tend to de-stratify the solution; the system would
be self-stirring. Such a system could be designed to work under any
container orientation.

[0037]In order to judge the feasibility of the above-described humidity
control device of the present invention, some standard form-factor
(3.5-inch) hard disc drives were fitted with containers sealed with
vapor-permeable membrane material. These containers had an internal
volume of approximately 2 cubic centimeters and were filled with an
aqueous solution of sodium sulfate (Na2SO4) or magnesium
chloride (MgCl2) containing surplus solid salt. After equilibrating
the hard drives at 25° C. and 40% RH, they were challenged by a
25° C. and 90% RH environment. These test disc drives were
instrumented with temperature and RH sensors so that the internal state
of the drives could be monitored. Results of these tests are presented in
FIG. 7.

[0038]Referring now to FIG. 7, it can be seen that the RH levels of both
the test drives fitted with prototype humidity control devices of the
present invention (plots 706 and 708) performed very well, relative to a
drives without an absorber (plots 702 and 704). Time-RH profiles (706 and
708) for the drives fitted with the humidity control devices of the
present invention are very flat showing that RH can be controlled within
tight limits.

[0039]FIG. 8 includes plots similar to those of FIG. 7. As can be seen in
FIG. 8, a test disc drive fitted with a prototype humidity control device
of the present invention having a humidity-controlling mixture including
activated carbon, PAA and K2CO3 (plot 802) performed substantially better
than humidity control devices of the prior art (plot 804 represents
results for a humidity control device including activated carbon and
silica gel-A and plot 806 represents a similar device including only
activated carbon).

Position of the Humidity Control Device

[0040]Referring back to FIG. 1, humidity control device 105 is shown
within housing 102 in a position proximate disc pack 106. In general,
humidity control device 105 can be mounted in any suitable position
within disc drive 100. For example, in some embodiments, humidity control
device 105 may be positioned such that it serves as the porous filter
that covers breathing hole 104 (FIG. 1).

Potential Benefits of the Solution

[0041]Control of RH levels within the drive should increase the
reliability by reducing the failure rate from mechanisms aggravated by RH
extremes. Control of RH levels within the drive should enable increased
performance of the recording subsystem by allowing a reduction in the
thickness of protective coatings on both media and heads, such as
diamond-like carbon, lubricant, and others. At a given nominal head
fly-height, a reduction in coating thickness will increase the read/write
signal strength relative to noise.

[0042]It is to be understood that even though numerous characteristics and
advantages of various embodiments of the invention have been set forth in
the foregoing description, together with details of the structure and
function of various embodiments of the invention, this disclosure is
illustrative only, and changes may be made in detail, especially in
matters of structure and arrangement of parts within the principles of
the present invention to the full extent indicated by the broad general
meaning of the terms in which the appended claims are expressed. For
example, the particular elements may vary depending on the particular
application for the humidity control system while maintaining
substantially the same functionality without departing from the scope and
spirit of the present invention. In addition, although the preferred
embodiment described herein is directed to a humidity control system for
a disc drive, it will be appreciated by those skilled in the art that the
teachings of the present invention can be applied to any enclosed system
within which humidity control is desired, without departing from the
scope and spirit of the present invention. In addition to the above-noted
salts, any salt or combination of salts that have one or more of the
above-listed properties can be utilized along with the superabsorbent
polymer, without departing from the scope and spirit of the present
invention. Although the preferred embodiment utilizes a container that is
at least partially formed of a vapor-permeable membrane, in general, any
material, through which water vapor can freely move, may be utilized.